1. Field of the Invention
The present invention relates to a bidirectional wireless communication system, a wireless communication apparatus, and a bidirectional wireless communication method. To be more particular, the present invention is applicable to a bidirectional wireless data transmission system that uses different carrier frequencies for transmission and reception in fast transmitting signals with carrier frequencies being in a millimeter band of 30 GHz to 300 GHz for carrying movie images and computer graphics, for example, between devices arranged relatively closely to each other or inside a device.
2. Description of the Related Art
With the recent enormous increase in the amount of information, such as movie images and computer graphics, demands have been increasing for systems capable of handling high-speed and large-capacity digital communication regardless of wired or wireless. Wireless data transmission systems based on millimeter waves have features for implementing high-speed data transmission. With these high-speed and mass-capacity wireless data transmission systems, the millimeter band has been attracting attention as a frequency band of carrier signals to be used. Accordingly, more and more wireless communication apparatuses for transmitting modulated signals of the millimeter band at high speeds have come to use.
Especially, use of a wireless communication apparatus, such as one mentioned above, for the communication inside a device (the communication between chips, between boards, or between modules inside a device, for example) eliminates the necessity for conductor-based transmission paths. Besides, the use of this wireless communication apparatus enhances the degree of freedom in the arrangement of boards for example, thereby lowering the mounting cost and overcoming EMI (Electro-Magnetic Interference) problems that are conspicuous in LVDS (Low Voltage Differential Signaling).
An attempt to build a bidirectional wireless communication system based on the two wireless communication apparatuses as described above demands a device realized in low power dissipation in the millimeter band, low cost, and small size. Especially, a frequency locking method in an RF (radio frequency) circuit is an important factor in any wireless data transmission system and the circuit scale of such a method heavily depends on how this method is implemented.
For example, according to bidirectional wireless communication systems widely used in the third-generation digital cellular system, wireless communication apparatuses each having a transceiver configuration having a transmitter block and a receiver block both based on frequency synthesizing are used.
Referring to FIG. 10, there is shown a block diagram illustrating an exemplary configuration of a related-art bidirectional wireless data transmission system No. 4. The system No. 4 is made up of two wireless communication apparatuses (hereafter referred to as TRX 40 and TRX 50) each using a frequency synthesizer.
The TRX 40 is configured by a reference oscillator 15, a PLL (Phase Locked Loop) circuit 16, a local oscillation circuit 17, a transmission section 18, a reception section 19, a transmission antenna 106, and a reception antenna 107. The reference oscillator 15 oscillates a signal having a reference oscillation frequency (hereafter referred to as a reference oscillation signal) to supply the generated oscillation signal to the PLL circuit 16.
The reference oscillator 15 is connected with the PLL circuit 16. The PLL circuit 16 has a frequency synthesizer function and generates an oscillation signal having two or more frequencies on the basis of the reference oscillation signal. The PLL circuit 16 is connected with the local oscillation circuit 17. On the basis of the oscillation signal having two or more frequencies, the local oscillation circuit 17 generates a carrier signal having two or more local oscillation frequencies.
The local oscillation circuit 17 is connected with the transmission section 18 and the reception section 19. The transmission section 18 is made up of a modulation block 101, a baseband amplifier 102, an up-conversion mixer 103, and a power amplifier 105. The modulation block 101 modulates entered data DIN1 into a modulated signal SIN1 and outputs the modulated signal SIN1 to the baseband amplifier 102. The modulation block 101 is connected with the baseband amplifier 102. The baseband amplifier 102 amplifies the supplied modulated signal SIN1. The baseband amplifier 102 is connected with the up-conversion mixer 103.
The up-conversion mixer 103 is connected with the local oscillation circuit 17. On the basis of the carrier signal output from the local oscillation circuit 17, the up-conversion mixer 103 up-converts the amplified modulated signal SIN1 and outputs the resultant signal Sout1 to the power amplifier 105. The up-conversion mixer 103 is connected with the power amplifier 105. The power amplifier 105 amplifies the transmission signal Sout1. The power amplifier 105 is connected with the transmission antenna 106, from which the amplified transmission signal Sout1 is radiated.
The reception section 19 is made up of a low-noise amplifier 108, a down-conversion mixer 109, a baseband amplifier 119, and a demodulation block 112. The reception antenna 107 is connected with the low-noise amplifier 108. The low-noise amplifier 108 amplifies a reception signal Sin2 received from the TRX 50. The low-noise amplifier 108 is connected with the down-conversion mixer 109.
The down-conversion mixer 109 is connected with local oscillation circuit 17. On the basis of the carrier signal output from the local oscillation circuit 17, the down-conversion mixer 109 down-converts the amplified reception signal Sin2 and outputs the resultant baseband signal SOUT2 to the baseband amplifier 119. The down-conversion mixer 109 is connected with the baseband amplifier 119. The baseband amplifier 119 amplifies the down-converted baseband signal SOUT2. The baseband amplifier 119 is connected with the demodulation block 112. The demodulation block 112 demodulates the amplified baseband signal SOUT2, thereby reconstructing the data DOUT2.
The TRX 50 is made up of a frequency offset detection block 27, a variable reference oscillator 28, a PLL circuit 29, a local oscillation circuit 30, a reception section 31, a transmission section 32, a reception antenna 201, and a transmission antenna 211. The frequency offset detection block 27 detects a frequency offset from demodulated data DOUT1 output from the reception section 31 and outputs a resultant frequency offset detection signal.
The frequency offset detection block 27 is connected with the variable reference oscillator 28. The variable reference oscillator 28 oscillates a reference oscillation signal to output the oscillated reference oscillation signal to the PLL circuit 29. The variable reference oscillator 28 is connected with the PLL circuit 29. The PLL circuit 29 has a frequency synthesizer function and generates an oscillation signal having two or more frequencies on the basis of the reference oscillation signal. The PLL circuit 29 is connected with the local oscillation circuit 30. The local oscillation circuit 30 generates a carrier signal having two or more local oscillation frequencies on the basis of oscillation signal having two or more frequencies.
The reception section 31 is made up of a low-noise amplifier 202, a down-conversion mixer 203, a baseband amplifier 218, and a demodulation block 205. The reception antenna 201 is connected with the low-noise amplifier 202. The low-noise amplifier 202 amplifies the reception signal Sin1 received from the TRX 40. The low-noise amplifier 202 is connected with the down-conversion mixer 203. On the basis of carrier signal output from the local oscillation circuit 30, the down-conversion mixer 203 down-converts the amplified reception signal Sin1 and outputs the resultant baseband signal SOUT1 to the baseband amplifier 218.
The down-conversion mixer 203 is connected with the baseband amplifier 218. The baseband amplifier 218 amplifies the down-converted baseband signal SOUT1. The baseband amplifier 218 is connected with the demodulation block 205. The demodulation block 205 demodulates the amplified baseband signal SOUT1, thereby reconstructing the data DOUT1.
The transmission section 32 is made up of a modulation block 206, a baseband amplifier 207, an up-conversion mixer 208, and a power amplifier 210. The modulation block 206 modulates entered data DIN2 and outputs a resultant modulated signal SIN2 to the baseband amplifier 207. The modulation block 206 is connected with the baseband amplifier 207. The baseband amplifier 207 amplifies the modulated signal SIN2.
The baseband amplifier 207 is connected with the up-conversion mixer 208. The up-conversion mixer 208 is connected with the local oscillation circuit 30. On the basis of the carrier signal output from the local oscillation circuit 30, the up-conversion mixer 208 up-converts the amplified modulated signal SIN2 and outputs the resultant transmission signal Sout2 to the power amplifier 210. The up-conversion mixer 208 is connected with the power amplifier 210. The power amplifier 210 amplifies the transmission signal Sout2. The power amplifier 210 is connected with the transmission antenna 211, from which the amplified transmission signal Sout2 is radiated.
Configuring the bidirectional wireless communication system No. 4 as described above can realize the enhancement and stabilization of the frequency accuracy of the local oscillation circuit 17 through the frequency synthesizer based on the PLL circuit 16 in the TRX 40. In addition, the enhancement and stabilization of the frequency accuracy of the local oscillation circuit 30 can be realized through the frequency synthesizer based on the PLL circuit 29 in the TRX 50.
The frequency accuracies of the local oscillation circuit 17 and the local oscillation circuit 30 depend on the reference oscillator 15 and the variable reference oscillator 28, respectively. The oscillation frequencies of the reference oscillator 15 and the variable reference oscillator 28 are different from each other between the TRX 40 and the TRX 50. Consequently, there resultantly occurs a difference between the oscillation frequencies of the TRX 40 and the TRX 50. In order to suppress this frequency difference, the frequency offset detection block 27 is arranged in the TRX 50 to control the oscillation frequency of the variable reference oscillator 28 to detect a frequency offset from the data DOUT1 obtained by demodulating the reception signal Sin1, thereby correcting the frequency difference.
For example, WCDMA (Wideband Code Division Multiple Access) demands the frequency accuracy of the variable reference oscillator 28 before the correction to be +/−2 ppm and the that after the correction to be +/−0.1 ppm. The TRX 40 uses a VCTCXO (voltage control oscillator of temperature compensation type) for the reference oscillator 15, thereby realizing these frequency accuracies in a certain temperature range (−25 degrees centigrade to +75 degrees centigrade, for example). The TRX 50 uses a pilot part of the reception signal Sin1 to detect a frequency offset to control the frequency control terminal voltage of the VCTCXO, thereby executing the fine tuning of the frequency. Thus, according to the frequency synthesizing, the absolute and relative accuracies of frequency can be controlled.
Further, a method is proposed in which, instead of the local oscillation circuit 17 and the local oscillation circuit 30 that use the PLL circuit 16 and the PLL circuit 29, for example, a free-running local oscillator of injection lock type using locking pull-in phenomenon may be used for the locking of transmission and reception frequencies. This phenomenon is described in Razavi, “A Study of Injection Locking and Pulling in Oscillators,” IEEE Journal of Solid-state Circuits, Vol. 39, No. 9, September 2004, (hereinafter referred to as Non-Patent Document 1), for example. Injection locking is widely known as a phenomenon in which, when a signal having a frequency in the proximity of an oscillation frequency is injected in an oscillator, the oscillation frequency of this oscillator is pulled in the frequency of the injected signal. To be more specific, a reference carrier signal is multiplexed with a modulated signal to be injected in a free-running local oscillator, thereby causing a pull-in phenomenon. This free-running local oscillator eliminates the necessity for the PLL circuit 16 and the PLL circuit 29, thereby realizing circuit simplification.
As for a wireless communication system based on the free-running local oscillator described above, Japanese Patent Laid-open No. 2007-158851 (page 5, FIG. 2) (hereinafter referred to as Patent Document 1) discloses a bidirectional wireless communication apparatus, a bidirectional wireless communication system, and a bidirectional wireless communication method. According to this bidirectional wireless communication system, when wireless modulated signals between bidirectional wireless communication apparatuses are transmitted or received, each bidirectional wireless communication apparatus is configured by transmission means, band separation means, injection lock oscillation means, and reception control means. The transmission means transmits a wireless modulated signal generated by multiplying an intermediate frequency band modulated signal obtained by modulating a signal to be transmitted to the mate of communication into an intermediate frequency band by a local oscillation signal.
The band separation means band-separates the harmonic component of the local oscillation signal having frequency N·fLO and the wireless modulated signal component from the received reception signal. The injection lock oscillation means divides the harmonic component of the local oscillation signal separated by the band separation means by 1/N to generate a local oscillation signal having frequency fLO.
On the premise of this, the reception control means multiplies the wireless modulated signal separated by the band separation means by the harmonic component of the local oscillation signal generated by the injection lock oscillation means, thereby down-converting a resultant signal into the intermediate frequency band. Configuring the bidirectional wireless communication system as described above can lower the frequency of the local oscillation signal, so that the manufacturing processes of the bidirectional wireless communication system can be simplified, eventually leading to a significant cost reduction.
In addition, Japanese Patent Laid-open No. 2007-228499 (page 7, FIG. 2) (hereinafter referred to as Patent Document 2) discloses a signal processing apparatus and method as an example of arranging a wireless communication apparatus of injection lock type inside a device. According to the disclosed signal processing apparatus, two or more signal processing blocks for processing data signals that are signals of predetermined data are arranged in one housing. Of these two or more signal processing blocks, a predetermined signal processing block has injection lock oscillation means.
In locking with an injection signal, the injection lock oscillation means oscillates such that a carrier signal for modulating or demodulating a data signal to be transmitted to any one of the two or more signal processing blocks in a wireless manner is generated. The injection signal (or a clock signal) comes from another signal processing block or the oscillator in a wired manner. The data signal is transmitted to any one of the two or more signal processing blocks in a wireless manner.
Each signal processing block has communication means. On the premise thereof, the communication means, by use of a carrier signal, modulates data signal to be transmitted to any one of the two or more signal processing blocks in a wireless manner or demodulates a data signal coming from any one of the two or more signal processing blocks in a wireless manner. Configuring each signal processing block as described above can efficiently execute data signal transmission and reception in a wireless manner within the housing of the apparatus.